Devices used for wireless communication are becoming smaller and smaller while at the same time communicating with still more wireless entities. A cell phone may for example have BLUETOOTH® connectivity, Wireless Local Area Network connectivity, FM radio connectivity, GPS functionality, etc. A hearing aid may provide connectivity not only to another hearing aid in a binaural hearing aid, but also to accessories such as cell phones, wireless remote controls, television sets, etc. The hearing aid may have connectivity to all of these entities either directly or via antenna dongles. Each of these connectivities requires an antenna for correct transmission and reception of signals. However, integrating two or more antennas in a small device typically leads to coupling between the antennas, and especially as the devices are being miniaturized.
Specifically for hearing aid users, the communication via mobile phones may be difficult due to interference between the mobile phone and the digital hearing aid. Therefore, it has been suggested that a hearing aid user uses the mobile phone without the hearing aid and with e.g. the volume control setting of a handset being at maximum value. Another solution has been suggested in which the mobile phone is inductively connected to the hearing aid, e.g. via a so-called Telecoil or T-link.
To ease the communication, one straightforward solution could be to place a Bluetooth receiver directly in the hearing aid for communication with a Bluetooth element in the mobile phone. However, it is not feasible to place a Bluetooth transceiver directly in the hearing aid device, as a Bluetooth transceiver would deplete the hearing aid battery too fast. It has therefore been suggested to use a Bluetooth bridging device having a proximity antenna for communicating with the hearing aid and a Bluetooth antenna for communicating with the Bluetooth transceiver in the mobile phone. However, as the proximity antenna and the Bluetooth antenna operate at the same frequency, i.e. around 2.4 GHz, strong interference between the proximity antenna and the Bluetooth antenna have been reported influencing both the signal quality and the connectivity.
Thus, for isolation among the antennas, antenna design and antenna placement in devices are becoming a still more important design factor. For closely spaced antennas configured to operate at different frequencies, the use of wavelength filters for reducing coupling has been suggested. However, such filters, typically LC filters, takes up too much space and tend to reduce bandwidth and efficiency for the antenna.
Also, for closely spaced antennas provided at a common printed circuit board, one or more slits in the printed circuit board have been suggested for providing isolation between the antennas. However, the efficiency of such a slit is often reduced by providing other conductors across the slit for connecting components on the printed circuit board on either side of the slit. Furthermore, providing a slit in the ground plane also reduces the effective ground plane for each antenna, thus reducing the antenna Q value.